Coatings, particularly those used in the food and beverage metal container industries, are expected to meet a number of requirements in order to be commercially viable. One of these requirements includes their ability to adhere well to the base metal over which they are applied. Another requirement is that they possess a certain degree of flexibility, extensibility and adhesion characteristics which enables them to withstand the fabrication of the container itself and/or the processing, if any, of the container's contents.
Another requirement of a coating which is designed for use in the metal container packaging industry results when the coating is applied in a manner such that it comes into direct contact with the consumer of the container's contents or the contents themselves. Under these circumstances, it is required that the coating not only be non-toxic, but also not adversely affect the taste of the food or beverage which is stored in the coated container.
Yet another requirement of a coating which is designed for use in the metal container packaging industry is the coating's ability to coalesce and/or form a continuous film. Specifically, if these properties are not present, the container's contents might be exposed to metal. This, in turn, can result in problems such as: prematurely corroding the container, contaminating the container's contents, adversely affecting the taste of the container's contents, and the like.
Still another requirement of coatings designed for use in the metal container packaging industry is that they resist "popping." The term "popping" is used in the coating industry to refer to a particular defect in coatings which develops during their curing process. Particularly during the curing process of many coatings, gaseous by-products are formed which can then become trapped within the coating. This problem most commonly occurs in areas where the coating has a relatively high film thickness.
Coating applicators typically need to take special precautions when working with coatings that are prone to popping so as to assure that a maximum allowable coating thickness is not exceeded on any portion of the article being coated. In some cases the speed at which coating is applied (i.e., the "line speed") is limited by a tendency for a coating to pop.
Even another requirement of coatings designed for use in the metal container packaging industry is that they resist "blushing." The term "blushing" is used in the coating industry to refer to another defect in coatings which develops during their curing process. Particularly, the term "blush" refers to a haziness in the film believed to be caused by absorption of water. This defect is particularly evident with container coatings that are subjected to high temperature, high pressure steam conditions during the canning retort process.
A defect that often accompanies blush is "blistering." The term "blistering" is used in the coating industry to refer to a sporadic lifting of the coating as salts, which are present at the interface of the metal substrate and the coating, are dissolved by water which penetrates the film.
As stated above, coatings designed for use on the interior of food and beverage containers should not adversely affect the taste of the container's contents. Taste problems can occur in a number of ways such as by leaching of coating components into the beverage, adsorption of flavor by the coating, a chemical reaction between the container's contents and the coating, the container's contents coming into contact with bare metal due to defects in the coating, and/or a combination thereof.
Since coatings designed for use on the end stock of metal containers are applied thereon prior to the ends being cut and stamped out of the coiled metal stock, in addition to all of the aforementioned properties, they must also be flexible and extensible. For example, can end metal stock is typically coated on both sides. Thereafter, the coated metal stock is punched, scored for the "pop-top" opening and the pop-top ring is then attached with a pin which is separately fabricated. The end is then attached to the can body by an edge rolling process. Accordingly, if used on coil metal stock from which can ends are to be made, the coating applied there over must have associated therewith a minimum degree of toughness and flexibility such that it can withstand extensive fabrication processes, in addition to being water and chemical resistant so as to prevent adversely affecting the taste to the container's contents.
The container industry frequently utilizes coatings which are based on epoxy resins to provide some of these properties. However, epoxy resins as sole film-forming vehicles do not adequately wet out the metal substrate and, therefore, fail to provide the desired level of coalescence and film continuity. For these reasons, container coating technology frequently uses epoxy-graft copolymers. An epoxy-graft copolymer is an epoxy resin which has been grafted with monomers such as styrene and methacrylic acid. It has been observed that such epoxy resins which have been modified with acrylic monomers impart improved coalescence and film continuity to the resulting coating. It has also been noted that merely blending an epoxy resin with an acrylic polymer yields a coating composition which lacks the same level of homogeneity and stability which is obtained with an epoxy-graft copolymer.
U.S. Letters Patent 5,252,669 to Maska et al. discloses a water reducible resin suitable for use as a coating for metal substrates such as metal food and beverage cans. The resin disclosed in that patent is produced by grafting in organic solvent a thermoplastic polymer (having no crosslinkable moieties) which contains grafting agents onto a thermosetting polymer (having crosslinkable moieties) which contains acid functional groups to render the resultant polymer water soluble. The thermoplastic polymer is concurrently grafted onto a polyepoxide which has previously been phosphatized. The resulting polymer is used as the resinous binder in a waterborne coating for use as a container coating.
U.S. Letters Patent 5,290,828 to Craun et al. discloses an aqueous, low volatile organic content ("VOC") coating containing a polymeric binder which includes an addition co-polymer grafted epoxy polyester terpolymer. According to that patent, the coating disclosed therein is suitable for spray application to metal substrates such as beer and beverage cans. The graft terpolymer includes an unsaturated polyester, an epoxy resin and an addition co-polymer grafted to the polyester. The addition co-polymer includes from between 20 to 100 percent carboxyl functional ethylenic monomers. The addition co-polymer grafted epoxy polyester has an Acid Number above 30 to facilitate dispersing the terpolymer into water using a volatile base. The epoxy resin of that patent is not phosphatized.
U.S. Letters Patent 4,212,781 to Evans et al. discloses a process for modifying an epoxy resin by reacting in organic solvent the epoxy resin with addition polymerizable ethylenically unsaturated monomers to produce a reaction mixture which includes an epoxy-acrylic co-polymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. According to that patent, in the modified epoxy resins which are suitable for use in waterborne coating compositions spray applied onto can coatings, the in situ polymerized monomer must include acid functional monomers to provide sufficiently high acid functionality in the reaction mixture to effectuate stable dispersion in water, although solvent vehicles may also be used. Neither the epoxy groups of the epoxy resin nor of the final reaction mixture disclosed in that patent are phosphatized.
U.S. Letters Patent 5,428,084 to Swarup et al. discloses an amine defunctionalized epoxy resin and coating compositions which contain such a defunctionalized epoxy resin which are suitable for application onto metal surfaces such as can end stock. The amine defunctionalized epoxy resin is prepared by reacting a polyepoxide with ammonia or an amine having at least two active hydrogens, using close to a 1:1 ration of equivalents of epoxy to equivalents of ammonia or amine. The reaction of the polyepoxide resin with the ammonia or amine involves a ring-opening reaction where the resultant ungelled product is the amine-terminated product of a polyepoxide resin. The coating compositions disclosed in that patent comprise a resinous blend of the amine defunctionalized epoxy with another resin such as a vinyl addition copolymer containing about 5 to 25 weight percent of an alpha, beta ethylenically unsaturated carboxylic acid to provide dispersibility to the copolymer. The resultant coating compositions are water reducible and can additionally contain a curing agent. Neither the epoxy groups of the epoxy resin nor of the final reaction mixture disclosed in that patent are phosphatized.
According to the specifications of the aforementioned U.S. Patents, the reaction products and respective waterborne coatings disclosed therein provide water-based dispersions that are suitable for spray application as a sanitary liner or inside spray coating for beer and beverage can interiors. In order to facilitate water dispersibility of these reaction products, skilled artisans know that it is necessary for these coatings to contain a minimum amount of acid functional monomer.
When acid functional monomers are present in an amount which facilitates dispersibility, their presence in this concentration also tends to promote hydrophilicity of the of the resulting coating. The hydrophilicity of these monomers is undesirable, especially when the coating is designed for use on the interior of metal food and beverage containers. Specifically, skilled artisans know that hydrophilicity may lead to increased water absorption of the cured coating. This, in turn, can result in blushing and/or blistering during processing procedures.
Additionally, many of the waterborne coatings of the type disclosed in the aforementioned U.S. Patents are not suitable for use as roll applied coatings. Accordingly, skilled artisans would not be inclined to roll-apply such coatings onto flat metal stock used for fabrication of can ends.
As demonstrated above, although there are a number of coating compositions which may be applied onto can end stock, most of the conventional coatings have a number of deficiencies associated therewith which require the formulator, applicator and/or processor to compensate therefor. A coating which minimizes and/or eliminates many, if not all, of the aforementioned deficiencies would be greatly welcomed by the metal packaging industry.